1. Field of the Invention
This invention relates to a fuel gas burner suitable for use in industrial furnaces, such as regenerative glass melting furnaces, which produces a flame comprising a solid core fuel jet and retards the formation of NO.sub.x. More particularly, this invention relates to a solid core fuel jet, low NO.sub.x fuel gas burner of a tube-in-a-tube design wherein the inner tube is remotely adjustable for varying fuel gas velocity so as to maintain optimal flame characteristics for flame control, low emissions, and maximum heat transfer.
2. Description of Prior Art
Industrial furnaces, such as regenerative glass melting furnaces, operate at extremely high combustion temperatures, typically in the range of about 2400-3000.degree. F. to promote higher production rates, higher product quality, and higher furnace thermal efficiency. As a result, furnace and flame temperatures tend to be high, resulting in the generation of significant amounts of NO.sub.x emissions. As a result of the 1990 Clean Air Act, many regional municipalities now impose NO.sub.x emission limits on high temperature industrial furnaces.
Recent efforts to address the increasingly stringent emission limits have resulted in the development of a few furnace retrofit NO.sub.x control technologies. The majority of these technologies operate to inhibit NO.sub.x formation by modifying flame stoichiometry and the overall combustion process. Such retrofit NO.sub.x control technologies include oxygen-enriched air staging in which oxygen-enriched air is introduced in stages into the combustion process, gas reburn in which a gaseous fuel is introduced into the flue gases downstream of the primary combustion zone, fuel staging in which the fuel is introduced in stages into the combustion process, oscillating combustion, and pulsed combustion. In oscillating combustion, the flow of fuel is oscillated around a stoichiometric value, thereby producing alternatingly fuel-rich and fuel-lean zones within the flame. Because both fuel-rich and fuel-lean combustion produces less NO.sub.x than stoichiometric combustion, the NO.sub.x formed in each zone is significantly lower than that which would occur if the combustion took place without fuel oscillation but at the same overall stoichiometry.
Pulsed combustion utilizes a cyclic combustion process to produce heat and pressure waves that increase a drying or heating rate. The combustion cycle begins when air and the fuel are ignited inside a combustor to produce a rapid pressure rise. The rise in pressure temporarily shuts off the lower pressure fuel supply. The combustion induced pressure then drops as the combustion products leave the combustion chamber, drawing a fresh air/fuel mixture into the chamber and restarting the cycle. Due in part to the success of the retrofit NO.sub.x control technology and the knowledge of the importance of proper furnace control in reducing emissions, increasing furnace efficiency, and extending furnace life, developments in the areas of improved furnace controls using advanced sensors, temperature mapping techniques, and computer control algorithms to supplement the primary combustion process can be expected. Eventually, a computer control system will couple the improved sensors to the burners and emission control system to enable maximum furnace efficiency, lowest possible emissions, and extended furnace life.
Before a control strategy can be coupled to burners, it is necessary to have a burner which can be remotely tuned. Conventional burners are equipped with either manual velocity adjustment mechanisms local to the burner or have no velocity adjustment capabilities whatsoever. Known burners with velocity adjustment capabilities utilize a tube-in-a-tube design to vary velocity in order to maintain optimal flame characteristics for flame control, low emissions, and maximum heat transfer. In such burners, gas flows through the annular space created by the tube-in-a-tube arrangement. The inner tube is provided with a tapered tip which translates manually through the fuel outlet of the outer tube, thereby varying the area of the fuel outlet of the outer tube and, thus, the gas velocity. However, such annular jets can be less stable than a solid core gas jet and are destroyed relatively quickly upon exit from the burner. Rapid destruction of the annular jet can cause an increase in mixing between the gas and air stream which, in turn, is known to increase NO.sub.x emissions. An industrial burner in which fuel and combustion air are provided through coaxial conduits into a combustion chamber is taught, for example, by U.S. Pat. No. 5,570,679 to Wunning. U.S. Pat. No. 5,139,416 to Wagner et al. teaches a gas burner for use in glass melting furnaces comprising a mixing tube having a perforated mixer for mixing fuel gas with air inspirated with gas flow, means for supplying fuel gas to the mixing tube, and an inspiration tube located in the mixing tube. The perforated mixer is situated inside the mixing tube at the end of the inspiration tube.
A significant complication in the manual adjustment of conventional burners, particularly on a routine basis, is their location on hot, difficult to access combustion ports. As a result, when burners are initially installed, they are tuned by adjustment of the velocities of the fuel gases flowing therethrough to provide optimal performance for the initial firing rate. In the case of glass melting furnaces, an optimal flame provides maximum heat transfer to the glass melt, low emissions, and good flame control so as not to impinge on refractory surfaces. However, when burner fuel flow rates are varied due to load changes, or for reasons of temperature control within the furnace, the burners are typically not readjusted due to inaccessibility or other inconvenience, and optimal flame characteristics are sacrificed. This is particularly true in situations where fuel changes occur frequently. Failing to maintain proper flame control can result in higher emissions, reduced fuel efficiency, and accelerated refractory wear. Similar detrimental results can occur when combustion air flow rates are changed, a requirement for using several known retrofit NO.sub.x control technologies.